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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Thu, 21 Mar 2019 00:44:08 -0400
Message-ID: <CADnq5_PTfJXJ99gFhxeMZ3RKigkipwjH5jLqfNM3mv49Z=d7Bw@mail.gmail.com>
Subject: Re: Clang warning in drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     "Koenig, Christian" <Christian.Koenig@amd.com>,
        "Zhou, David(ChunMing)" <David1.Zhou@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "clang-built-linux@googlegroups.com" 
        <clang-built-linux@googlegroups.com>,
        "amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
        "Deucher, Alexander" <Alexander.Deucher@amd.com>,
        Nathan Chancellor <natechancellor@gmail.com>
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On Wed, Mar 20, 2019 at 4:49 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Wed, Mar 20, 2019 at 2:37 AM Koenig, Christian
> <Christian.Koenig@amd.com> wrote:
> >
> > Am 20.03.19 um 05:34 schrieb Nathan Chancellor:
> > > On Wed, Mar 20, 2019 at 01:31:27AM +0000, Pan, Xinhui wrote:
> > >> these two enumerated types are same for now. both of them might change in the future.
>
> Please consider if the YAGNI principle applies here.
> https://martinfowler.com/bliki/Yagni.html
>
> > >>
> > >> I have not used clang, but would  .block_id =  (int)head->block fix your warning? If such change is acceptable, I can make one then.
>
> My preference on solutions, in order:
> 1. One enum (this is the simplest most type safe solution).  Add
> another enum when you actually need it.  Otherwise, YAGNI.

It make sense to have two enums.  One is a firmware interface that is
only used by some asics and the other is for the general driver
interface (non-asic specific for the ras features.

> 2. Safe casting function (like the one Nathan supplied, maybe with
> WARN_ONCE rather than WARN).  This ensures that at least if the types
> diverge you get a runtime warning.  A compile-time warning would be
> preferred, but I haven't taken the time to think through how that
> might be implemented.

I'd prefer this one.

Alex

> 3. Cast to int (this has been used in other places throughout the
> kernel, but provides the weakest type safety and doesn't catch future
> divergence).
> 4. Disabling the warning. (I almost never advocate for this).
>
> > Another question is if it is such a good idea to just silence the warning?
>
> For the kernel, it seems that each maintainer can choose what to apply
> to their subsystem.  I would recommend against disabling additional
> warnings that aren't disable kernel-wide for most cases.
> -Wenum-conversion has spotted many bugs.  While the enums in question
> today are not different, they MIGHT eventually diverge and lead to
> bugs, like the others we've found and fixed throughout the kernel.  So
> I would recommend fixing now, and be insulated in the future.
>
> --
> Thanks,
> ~Nick Desaulniers
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